Fix accidental backward-incompatible change (bug#62417)

Fix accidental backward-incompatible change (bug#62417)

This code used to work, but with the change of 59ecf25fc860 it stopped
working:

   (defun foop (buffer-name _alist) (string-match "foop" buffer-name))
   (add-to-list 'display-buffer-alist '(foop . display-buffer-other-frame))

This change makes it work again, restoring compatibility.

* lisp/subr.el (buffer-match-p): Fix and adjust docstring.
* lisp/window.el (display-buffer-alist): Adjust docstring.
(display-buffer-assq-regexp): Make good on promise of display-buffer-alist.